This invention relates to a method for preparing alkaline earth metal carbonates, particularly calcium carbonates, with improved properties for use as fillers or pigments in paper, paints, plastics, and household cleaners. Most particularly, this invention relates to improving the properties of natural ground calcium carbonate suspensions for use as fillers in paper and paints.
Many paper mills have converted to neutral or alkaline papermaking in the past two decades because this has offered them a number of advantages; among these is the opportunity to reduce pigment costs by replacing some of the expensive titanium dioxide used as a filler in papermaking with less expensive calcium carbonate pigment. The term "filler" is employed to refer to a pigment which is admixed with wood pulp in an aqueous suspension immediately prior to the dewatering of the pulp and pigment admixture to form a sheet of paper. Calcium carbonate, particularly precipitated calcium carbonate (PCC), is being used as a filler pigment in the making of paper; natural ground calcium carbonate (GCC) is being used in North America predominantly as a coating pigment while it is also used as a paper filler pigment in Europe. Both types of calcium carbonate pigments lack the physical properties which would allow papermakers to completely eliminate the use of more expensive pigments such as titanium dioxide or synthetic amorphous silicas and silicates. The GCC products are generally considered to be inferior to scalenohedral PCC for use as fillers for paper because their generally spherical particle morphologies do not scatter light as efficiently as the rosette-shaped PCC particles which contain many microvoids.
The spherical GCC particles pack together very efficiently and can be dispersed to form a fluid suspension at relatively high solids concentrations. GCC is sold to the paper industry predominantly as an anionically-dispersed 75% to 76% solids suspension of fine particles because this form is free of dust, can be pumped, requires a small storage volume compared to other forms, and is less energy-intensive to produce and distribute than other forms.
PCC is generally provided to the paper mill as a dry powder or as a 20% to 30% aqueous suspension from a "satellite" PCC production facility located immediately adjacent to the paper mill.
Economic benefits would be realized by the paper manufacturer if scalenohedral PCC could be inexpensively modified to allow it to be used as a greater proportion of the overall filler pigment admixture within speciality papers. Economic and environmental benefits would be realized if GCC could be inexpensively modified to make it an acceptable paper-filling pigment when used at high filler levels in printing and writing papers; an evaluation conducted as a result of EC Directive 1836/93, the EC Eco-Audit Directive, found GCC to be less energy intensive in its production and distribution to the paper mill than PCC.
Improvement in the specific pigment physical properties of ink absorption, opacity, and first pass retention (retention of the pigment in the paper sheet when it is formed from the pulp suspension) would allow papermakers to use a greater proportion of calcium carbonate filler in paper thus reducing their use of other, more expensive fillers.
In addition to prior art concerning the production of both GCC and PCC pigments, several patents teach modification of the surface of calcium carbonate particles to make them more acceptable for use as filler pigments under weakly acidic papermaking conditions.
A variety of techniques to modify calcium carbonate to achieve a degree of acid resistance have been patented. U.S. Pat. No. 5,037,477 discloses precipitating a coating on calcium carbonate particles by simultaneously adding to an aqueous suspension of calcium carbonate particles, at an elevated temperature, a solution of a water-soluble silicate and a solution of a water soluble zinc compound so as to precipitate a coating onto the particle surfaces very rapidly; U.S. Pat. No. 5,043,017 discloses adding a calcium chelating agent or a conjugate base and a weak acid into an aqueous suspension of calcium carbonate particles such that the calcium carbonate particles are coated by and in equilibrium with the additives; U.S. Pat. No. 5,531,821; 5,584,923; 5,593,488; 5,593,489; 5,599,388; and 5,647,902 also disclose processes for rendering calcium carbonate particles more acid-resistant but do not employ silica.
U.S. Pat. No. 5,514,212 teaches the use of a calcium carbonate pigment which has a starch-soap complex precipitated onto the particle surface so as to improve the efficiency of sizing agents added to the wood pulp and pigment admixture during the papermaking process.